Measles remains a significant public health threat despite the widespread availability of a safe and effective vaccine. This research specifically attempts to define the timing and pattern of cytokine responses to measles vaccine. The overall objective of this type of research is to determine the role and pattern of cytokines operative in the immune response cascade to measles immunization and to use this information to better understand the factors which impact measles vaccine immunogenicity. The specific aims of this pilot study are to determine when the peaks of interferon gamma (IFNY), interleukin-2 (IL-2), interleukin-4 (IL-4), interleukin-6 (IL-6), and tumor necrosis factor alpha (TNF alpha) occur following measles immunication, and to explore the relationship between cytokine levels and measles antibody response. The time course and secretion profile of cytokines will be compared in two groups of children presenting for immunization. Group 1 will be 12 to 15 month olds receiving their first dose of measles vaccine and Group 2 will be 4 to 6 year olds receiving their second dose of measles vaccine. Forty children will be recruited from each group and then randomized to undergo phlebotomy on a specified day, either on day 0 (day of vaccination) or day 2, 5, 10, 15, 20, 30 or 40 after immunication with measles vaccine. Five children will have phlebotomy on any specified day. Measles antibody testing will be performed on samples obtained on days 30 and 40. For each sample collected, peripheral blood mononuclear cells will be isolated and frozen for culture at a later day. Cytokine assays will be performed in batches on supernatant fluids obtained from PBMC cultures, using commercially available ELISA kits. Data analysis will be performed using polynomial regression.